The invention relates to a method for comminuting and cleaning of waste plastic. The invention relates also to preparing the comminuted and cleaned plastics for various uses in the different grain size distribution curves. Finally, the invention also relates to the recovery of types of plastic from the comminuted mixed plastics.
With currently existing collection systems for waste plastics, e.g., packaging material from private households, yoghurt containers, protective wrappers, shopping bags, containers for cleaning products, toothpaste tubes, etc., mixed plastics are obtained at the end of the sorting chain. These include, along with the usual films and plastics composed of LDPE, HDPE, PS or PP, often film remnants composed of LDPE/HDPE coated with polyamides or PET films (compound films), polycarbonates, PET, polystyrene or other plastics. The quantitative material distribution of the various plastics, which are delivered to processing and sorting companies by collection companies, depend on, among other things, the collective behavior and quality awareness of the population. Despite advanced sorting, a complete separation of the various types of plastics is not attainable. Along with household collection systems, there are, however, also commercial collection systems exclusively for LDPE and HDPE plastics, with which predominantly plastic films are collected, which are produced using a sheeting calendar. Here too, increasingly mixed plastics, among others, compound films, accumulate. Furthermore, large quantities of packaging plastics are present in the ordinary household trash, most notably in urban areas. These mixed plastics collected there together with the household trash are currently separated on a trial basis, however, they are not yet systematically separated out and sent to recycling.
After an optimal cleaning and presorting, the mixed plastic (MKS) from collection systems are comminuted to flakes or scraps, after the so-called bottle fraction (e.g., laundry detergent bottles composed of polypropylene), but also large format film packaging have been separated out. The average “German” mixture from the “yellow bag” collection system is composed of 60-70% polyethylene, 5-10% polypropylene, 10-15% polystyrene and 5-10% of other plastics, The portion of PVC is less than 5%, with a decreasing trend.
Mixed plastic is naturally composed of more than one type of plastic, which can be separated to pure plastic only at a high expense, or not at all. According to this definition therefore, mixed plastics are also compound films, in which, e.g., an LDPE film is coated with a thin polyamide film or PET film. Currently, mixed plastics originate predominantly from the collection of packaging plastics. Mixed plastics are predominantly film remnants. However, mixed plastics can also contain chips from thick-walled containers or lids of injection molded parts. Thick-walled containers can also be film coated with protective films or similar, and are, therefore, mixed plastic.
Film scraps can not be ground up at all, or only unsatisfactorily. In many cases, the films are thinner than the grinding gap between the grinding discs, for instance, of a refiner, such that these pass through grinding process without being frayed. Only a compacting of the two-dimensional scraps into a three-dimensional agglomerate enables a satisfactory grinding process. This compacting optimally occurs in that the film remnants are formed into a wad by heating and at least partially surface-melted. This can occur together with chips from thick-walled plastic parts and/or together with other chip material or fiber material. A proven technology for this is the so-called pan agglomerator, which leads to the increase in a transport-optimal apparent bulk density in the form of a granular agglomerate. However, other methods for compacting film scraps into an agglomerate are also conceivable. Thus, for example, a targeted heating, which does not exceed the melting point of the carrier film, leads to a shrinking of the flakes, which in the process are compacted into an agglomerate, on their own, or moved in a drum, or passed through in a discharge chute by hot air.
It is essential for one aspect of the invention that the diameter of the created agglomerate does not exceed the distance of the grinder ribs of the grinder fittings of a refiner, because otherwise, this would immediately lead to a jamming of the intermediate space between the ribs. To prevent smaller material from slipping through between the grinder ribs, according to the invention, barriers can be built in between the ribs in the form of transverse webs. During the grinding process, the material is forcibly fed at these barriers into grinder gap between the grinding discs. These barriers are arranged with the disc refiners, as well as, also with cylinder or drum refiners at the appropriate location of the grinder fittings. In the simplest case, these barriers are welding spots; in the ideal case, the barriers are shaped out of the solid material during milling of the grinder fittings. The arrangement of the barriers depends on the type of the refiner, the distance of the grinder ribs and other parameters. The optimal arrangement is determined very simply on a trial basis by placement of welding spots, which can be removed again. The thusly attained grinder result is evaluated, and the final arrangement of the barriers is fixed on this basis.
In order to attain a sufficient transport bulk density, with the invention the flakes or scraps are compacted into a granular agglomerate. Depending on the purity and sorting, the granular stock is very uniform (with a high degree of purity control of the types of plastic), or it has a very non-uniform structure in both the appearance as well as in the consistency and the geometry of the agglomerate (high mixing of various types of plastic and different sources of origin, e g,., mixed plastics from household collection systems). During the processing, the mentioned agglomerating, the flakes or scraps are heated, e.g., in a pan agglomerator through friction by means of an agitator, such that they start to melt. By spraying the heated, melted plastic mass in intervals with cold water, a part of the organic components escapes via the water vapor. At the same time that the melted film pieces cool off, they are broken by the rotor blades of the agglomerator, and agglomerated into pourable, granular-like bodies. This agglomerate has a relatively high bulk density of approximately 250 to 350 kg/m3 and can be transported very easily. This method is described in DE 198 01 286 C1, the entire contents of which is incorporated herein by reference. However, there are also so-called disc agglomerators, which attain similar effects with the necessary compaction of the material. Other methods of compaction such as melting of flakes in a hot air stream or in a heated mixing drum, or also the compaction into pellets using sieve matrices are also possible.
In U.S. Pat. No. 5,154,361, the entire contents of which is incorporated herein by reference, a method is described in which film flakes, in particular, large format film remnants together with pulp are to be comminuted in a refiner, wherein a suspension is supplied beforehand, in which the plastic film, e.g., from shipping envelopes can be pre-comminuted in a Valley beater. This suspension is treated subsequently with additives and antifoam reagents, which are to prevent foam formation of solute adhesives, and shredded in a disc refiner in the presence of pulp. This method concerns especially the so-called rejects that accumulate in large quantities in the treatment of recovered paper, that is, pulp of recovered paper contaminated by plastic. The disclosed method has the purpose to pre-comminute the film flakes through pretreatment, such that together, with the pulp they form a fiber pulp. This is then ground up, such that pulp and plastic fibers are contained in this pulp in nearly equal size. Plastic films as rejects from the recovered paper are always tainted with a quantity, more or less, of cellulose fibers. As a result, the recycling industry would rather not accept these materials, because due to the moisture of the pulp fibers, e.g., they do not permit satisfactory agglomeration. Despite the high cleanness of the film, the pulp adhesion makes melting into a regrind economically impractical. The goal of the disclosed method is to improve the core of gypsum boards with this fiber material composed of plastic with additions of pulp. The method is described for very low material consistency, as is known from the grinding of cellulose pulp, that is, namely a maximum of 4.25%. The material is not first compacted into agglomerate. Because pulp is systematically provided in the process, the consistency for the cellulose pulp must not be exceeded.
In DE 103 30 756, the entire contents of which is incorporated herein by reference, a method for the production of a fiber material is disclosed, in which method a second group is mixed into a first group of fibers, where the second group is composed of plastic particles or fibers, which are obtained by comminuting and/or shredding of agglomerates of pure or mixed plastics in a disc refiner. Water is supplied to the disc refiner during the comminuting. The grain size of the plastic particles and/or plastic fibers corresponds approximately to the grain size of the particles of fibers of the first group.
The disclosed method serves, above all, for the production of molded parts composed of wood materials, wherein a portion of the wood fibers are substituted by plastic particles and/or plastic fibers. Here, but with other uses, a specific size of particles or fibers is of great importance. With the disc refiners, despite a narrow gap width it can occur, that a certain undesired portion of relatively long and large fibers or large grain chips are created from MKS. The cause of this lies in the fact that with a disc refiner, the ground stock is subjected to a strong centrifugal force due to high speed of the rotors, and thereby relatively large dimensioned particles can slide through between the grinding discs in the rib troughs of the grinder fittings to the outside. Barriers between the rib webs would be associated with the risk of blockages, because, e.g., wood chips are also ground up as the second material component. Therefore, it is necessary to add to this type of comminuting process a sorting process or separation process, using which the relatively large plastic fibers are separated out. Furthermore, the method described there, uses a refiner operated under an atmospheric pressure or under steam pressure, in which the ground stock is supplied via a screw conveyor (plug screw) to the refiner.
The objective of the invention is to specify a method for comminuting and cleaning waste plastic, especially mixed plastic (MKS), which open up a plurality of recycling possibilities of the processed waste plastics.